Modern light emitting diode (LED) lights are usually driven from constant current output drivers. In order to maintain the same LED current while the supply voltage changes, the input power to the light remains constant, hence the impedance of the light changes with supply voltage.
However, the intrinsic input impedance variation with supply voltage can cause problems. For example, such lights cannot be connected in series. If two lights were to be connected in series, a severe voltage imbalance will occur as one light becomes a low impedance and the other lights become a high impedance. One light will likely dim, while other lights end up with a high voltage across it. Depending on how high the supply voltage is, this may damage one of the lights. Due to the inevitable voltage imbalance problems, this type of light cannot be wired in a “Y” with floating neutral configuration, across a three phase supply, even if the loading per phase appears to be balanced.
For the above reasons, such lights can be damaged when connected to three phase systems and a neutral fault occurs (i.e., when the neutral to which the lights are connected becomes detached from the ground). Such faults are common in industrial lighting systems during installation and can occur at any time after installation. This results in a large voltage appearing across some of the lights on the circuit, which may exceed their rating, thereby, causing failure.
Universal input lights place additional strain on the power distribution systems when a low voltage condition occurs. This is because as the supply voltage drops they will continue to draw the same power, while increasing their input current. This is unlike traditional filament bulbs, for example, that will do the opposite during low voltage conditions.